Low chromium oxidation resistant austenitic stainless steel

Abstract

Nickel-chromium stainless steel is of austenitic composition specially controlled to enable achieving resistance to elevated-temperature oxidation and corrosion at desirably economical levels of alloy content as low as 10% chromium and 10% nickel. Oxidation and corrosion resistant characteristics of the steel particularly include resistance to air-water atmospheres and gasoline exhaust atmospheres that are cyclically heated and cooled with heating to temperatures as high as 1800° F. and cooling to room temperature. Steel has special utility for automotive exhaust train components and is generally useful for cyclically heated structural articles.

I claim: 
     
       1. An austenitic nickel-chromium stainless steel alloy consisting essentially of 10% to 15% chromium, about 10% to about 18% nickel, 0.5% to 4.5% silicon and 0.5% to 4.5% aluminum in amounts providing a total silicon-plus-aluminum content of 2% to 7% and correlated with the chromium content to provide that   % Cr+2(%Si+%Al) = 19 to 24,     0.02% to 0.15% carbon, up to 0.7% titanium, up to 2% manganese, up to 0.05% magnesium and balance essentially iron in an amount at least 60% of the alloy.   
     
     
       2. An alloy as set forth in claim 1 wherein the aluminum content is at least 2%, the silicon-plus-aluminum content is at least 3% and wherein the amounts of chromium, silicon and aluminum are in accordance with the relationship   % Cr+2(%Si)+%Al equal at least 18.     
     
     
       3. An alloy as set forth in claim 1 wherein the chromium content is greater than 14%, the aluminum content is at least 1% and wherein the amounts of chromium, silicon and aluminum are in accordance with the relationship   % Cr+2(%Si)+%Al = 16 to 23.     
     
     
       4. An alloy as set forth in claim 1 containing 10% to about 14% chromium and about 10% to about 14% nickel. 
     
     
       5. An alloy as set forth in claim 1 containing about 12% chromium, about 12% nickel, 2% to 2.5% silicon and about 2% aluminum. 
     
     
       6. A stainless steel wrought product made of the alloy set forth in claim 1 and having a worked microstructure wherein at least 80% of the structure is austenite. 
     
     
       7. A stainless steel product as set forth in claim 6 including in the form of a cold-work sheet, strip, bar or wire. 
     
     
       8. An austenitic stainless steel casting of the composition set forth in claim 1 and having a cast microstructure comprising as-cast dendrites of austenite dispersed in a nickel-chromium matrix. 
     
     
       9. An automotive exhaust train component made of the alloy set forth in claim 1.